This invention relates to a process for making blends of polycarbonates with certain polyamides, which blends have high notched Izod impact strength and frequently also an unexpectedly high flexural (or flex) modulus.
It is well known in the polymer industry to make polymer blends that have improved properties, when compared with the properties of the individual blend components. For example, it is a common practice to blend engineering resins such as, for example, polyamides, polyesters, or polycarbonates with low flex modulus, soft, rubbery polymers in order to improve their toughness (or impact strength). U.S. Pat. Nos. 4,172,859 and 4,174,658, both to Epstein, represent what is believed to be the most successful earlier work in this area. For the purpose of this invention, a polymer blend which has a notched Izod impact strength of at least 10 ft.lb/in (534 J/m), as determined according to ASTM Standard D256, is considered to be "supertough". Most Epstein blends are supertough.
While the starting engineering resins have high flex modulus, the flex modulus of the toughened engineering resins ordinarily is decreased, sometimes significantly. In some applications, this is not considered a drawback, but in some other applications such as, for example, machine parts or automobile parts, this would be a drawback.
Blends of crystalline polyamides, such as, for example, nylon 6 with polycarbonates also are known; see, for example, U.S. Pat. Nos. 4,782,114 to Perron and 5,008,374 to Thill. Such blends normally also contain a third and in some cases a fourth component, which is a polymeric compatibilizing agent, having low glass transition temperature and low flex modulus. Those blends have very high impact strength, but their flex modulus always is lower, sometimes significantly so, than that of the starting polyamide. The starting polycarbonate itself most often has a flex modulus that is lower than that of the polyarnide, and the flex modulus of the blend can be lower than that of the polycarbonate as well, so that in this case, the final composition may not have sufficient stiffness for those special applications.
It is customary in the industry to make polymer blends at the lowest temperature at which they can be melt processed, usually about 10.degree.-60.degree. C. above the melting temperature of the highest melting polymer. In the case of nylon 6, which melts at about 220.degree. C., the processing temperature normally is no higher than about 260.degree. C., although Perron describes operations at temperatures as high as 287.degree. C.
Certain aliphatic polyamides such as, for example, nylon 11, nylon 12, and nylon 12,12, have rather low flex moduli, which would be considered marginal in many engineering resin applications. Those polyamides can be readily toughened to high values, but their stiffness normally decreases below acceptable level. Blends of such polyamides with polycarbonates and toughening agents do not have a satisfactory toughness/stiffness combination.
There is a need, therefore, for a blend of such polyamides with a polycarbonate that would not only have high impact strength but also high stiffness.